1. Field of the Invention
The present invention generally relates to a wavelength division multiplexing device, an optical leakage prevention method, and a wavelength division multiplexing communication system, and especially relates to a wavelength division multiplexing device, an optical leakage prevention method, and a wavelength division multiplexing communication system that can adjust the optical power level.
2. Description of the Related Art
In order to maintain transmission quality of a wavelength division multiplexing communication system, it is important to reduce variations in the optical power level due to, for example, attenuation variation of optical parts and transmission lines, and gain variation of optical amplifiers. Especially, where the wavelength division multiplexing communication system is constituted by a great number of relays, tilts (variations of the optical power level by wavelength) are accumulated, and the wavelength variation of OSNR (optical S/N ratio) at a receiving end increases. Since degradation of OSNR causes an adverse influence on the system performance of the wavelength division multiplexing communication system as a reduction in the span length over which an optical signal can be transmitted, variations in the optical power level by wavelength have to be reduced.
As a method of reducing the variation in optical power level, a method of controlling and reducing the amount of the tilt is conceivable, wherein the tilt of the optical amplifier is compensated for by applying a tilt in an opposite direction of the tilt generated in the transmission line. Further, in the case where the wavelength division multiplexing communication system uses a distributed Raman amplifier (DRA), a method of controlling the optical power level of the excitation light of the DRA and compensating for the tilt is available. Either method is suitable for reduction of the tilt, such as reduction of a primary inclination.
FIG. 1 is a block diagram of an example of an OADM (Optical Add/Drop Multiplexing) node that constitutes a wavelength division multiplexing communication system. The OADM node is an example of a wavelength division multiplexing device. In the OADM node of FIG. 1, the variation in optical power level is reduced by the following methods.
According to a conventional OADM node (henceforth a node), a received optical signal is divided by wavelengths by a splitter 100. The divided optical signal is output to an adder 150 through a coupler (CPL) 120, after the optical power level is adjusted by a variable optical attenuator (VOA) 110. The coupler 120 divides the output of the variable optical attenuator 110 and sends one of the divided parts to a monitor photo diode (PD1) 130.
A control unit 140 analyzes the optical power level of the output of the variable optical attenuator 110 using the monitor photo diode (PD1) 130, and controls the magnitude of attenuation of the variable optical attenuator 110 so that the optical power level of the signal output from the variable optical attenuator 110 becomes a predetermined level (constant level control). According to this method, the optical power level of the signal output from the variable optical attenuator 110 is finely adjusted for every channel, and the variation in optical power level is properly reduced.
FIG. 2 is a control flowchart of the Node of FIG. 1. At Step S100, the control unit 140 controls the magnitude of attenuation of the variable optical attenuator 110 so that the optical power level of the signal output from the variable optical attenuator 110 becomes equal to a target VOA output level L1. The control unit 140 repeats Step S100 until the optical power level of the signal output from the variable optical attenuator 110 becomes less than a disconnection-detection threshold Th_d (i.e., so long as the condition is NO at S101). If the optical power level of the signal output from the variable optical attenuator 110 becomes less than the disconnection-detection threshold Th_d (i.e., YES at S101), the control unit 140 determines that the optical signal is no longer input (optical signal off), and transitions to an “off-state”.
During the “off-sate”, the control unit 140 controls the variable optical attenuator 110 so that the magnitude of attenuation of the variable optical attenuator 110 is set to a predetermined “off-state” attenuation A1 that is set at a magnitude of attenuation that can detect an input optical signal when the optical signal is resumed from the “off-state”. If the optical power level of the signal output from the variable optical attenuator 110 becomes greater than a recovery threshold Th_r (i.e., YES at S103), the control unit 140 transitions from the “off-state”, and resumes processing at Step S100. That is, the control unit 140 controls the magnitude of attenuation of the variable optical attenuator 110 so that the optical power level of the signal output from the variable optical attenuator 110 serves as the target VOA output level L1.
Patent Reference 1 discloses a technology wherein a variable optical attenuator of a node is controlled so that it autonomously reduces variations such that an optical surge, and the like, does not occur when a system becomes faulty.
[Patent Reference 1] JPA 2004-7058